1. Field of the Invention
The present invention relates to a wavelength selective optical switch for branching a signal light of arbitrary wavelengths for each wavelength at a node in a large-scale photonic network to which a plurality of wavelength division multiplexing networks is connected.
2. Description of the Related Art
With the rapid diffusion of high-speed access networks having bands of several to 100 Mbit/s or so, such as recent FTTH (Fiber to The Home) or ADSL (Asymmetric Digital Subscriber Line), an environment capable of enjoying broadband Internet services is being developed. In order to deal with an increase of the telecommunication needs thereof, in the backbone network (core network), the laying of ultra-large capacity communication system using the wavelength division multiplexing technology is being progressed.
On the other hand, in a connecting portion between the metro-network and this core network, since a limit of switching ability by the electricity still remains, an occurrence of band bottleneck in this portion is feared. Therefore, it is considered to be effective that a new optical switching node is installed in the metro region where the band bottleneck occurs, to construct the new photonic network architecture for directly connecting the access network and the core network using an optical region, without using the electric switch.
A function of selecting to switch a particular wavelength from one fiber has been treated as important as a function of the optical switching, and a switching device for realizing such a function is called a wavelength selective optical switch. As specific applications of this wavelength selective optical switch, there are for example, a wavelength selective optical router for controlling and routing signal lights of individual wavelengths from an input fiber to an output fiber, a wavelength selective optical node bypass for controlling to bypass the particular wavelength from one fiber to an alternate fiber, a wavelength selective add/drop for controlling the adding/dropping of the particular wavelength to/from one fiber.
As a conventional wavelength selective optical switch, as shown in FIG. 23 to FIG. 25 for example, there is known the one comprising a signal light input port 1 and a plurality of signal light output ports 2, each consisting of a collimate lens 3 and an optical fiber 4, a diffraction grating 5, a lens 6 and a mirror array 7 consisting of a plurality of micro-mirrors 8 (refer to U.S. Pat. No. 6,549,699 and Japanese National Publication No. 2003-515187). Note, FIG. 23 is a perspective view of the conventional wavelength selective optical switch, FIG. 24 is a top view thereof and FIG. 25 is a side view from A-direction of FIG. 24.
This conventional wavelength selective optical switch realizes the above described wavelength selective optical switch function, based on the operational principle in which parallel signal lights (collimate beams) of different wavelengths of M in number, which are emitted from the signal light input port 1, are separated to different angular directions by the diffraction grating 5, and thereafter, are condensed on different positions by the lens 6 (refer to FIG. 23 and FIG. 24), and the respective optical beams are reflected at desired angles by the mirror array 7 consisting of the M micro-mirrors 8 having angles variable due to the electrostatic attraction or the like, which are arranged on condensing positions of the respective optical beams passed through the lens 6, to be led to desired signal light output ports 2 (refer to FIG. 23 and FIG. 25).
At this time, the respective micro-mirrors 8 of the mirror array 7 need to be angularly adjusted so that the optical beams return the desired signal light output ports 2. Therefore, there has been conventionally used, for example, a method of detecting the angles of respective micro-mirrors 8 with the capacitance or the like to perform a feedforward control, a method of performing a feedback control so that the output intensity at the signal light output port for the desired wavelength becomes maximum, or the like. However, in the former feedforward control, there is a problem in that the micro-mirrors 8 cannot retain optimum angles if the capacitance is changed with time. Therefore, the latter feedback control is generally used.
In the case where the wavelength selective optical switch is operated in the actual network, such a case where signal lights of all reactive wavelengths (channels) are necessarily operated from an initial stage of the service-in is rare. As shown in an optical path indicated by the broken line in FIG. 24, there are many cases where the wavelength which is not in the service-in (dark channel) exists. In such a case, even when the signal light of the wavelength which has not been in service-in is newly in the service-in, the wavelength selective optical switch needs to appropriately start the switching operation without the interference or the crosstalk. Further, in the applications such as the wavelength selective optical router and the like, there is a case where the wavelengths or the number of wavelengths of the signal lights input to the wavelength selective optical router are frequently changed according to the switching status of routes. Even in such a case, needless to say, the wavelength selective optical router needs to operate without the interference or the crosstalk.
However, in the configuration of the above described conventional wavelength selective optical switch, if there exists the wavelength (dark channel) though which the input signal is not input to the wavelength selective optical switch, since any light does not reach the micro-mirror 8 by which the optical beam of this wavelength should be originally reflected, it is impossible to detect the angle of the micro-mirror 8 with the optical output intensity. Therefore, the feedback control of the angles of the micro-mirrors 8 as described above cannot be performed, and also, the information on the states of the angles of the micro-mirrors 8 cannot be obtained. FIG. 26 is a side view similar to FIG. 25, and shows by the broken line a route passing the micro-mirror 8 corresponding to the dark channel under the above status. Then, if the signal light of a new wavelength is input to the wavelength selective optical switch under this status, as shown by the solid line in FIG. 27, there is a possibility that the light is output to an unintended signal light output port. Therefore, there is a possibility of a serious problem of the occurrence of crosstalk.